Some turbine hardware, such as turbine airfoils, operate under extreme conditions. Turbine airfoils, blades for example, operate in an oxidative, corrosive atmosphere of hot exhaust gases at temperatures that may approach or exceed the melting temperature of their base material, which base material is typically a high cost superalloy. The base material usually is covered with bond coating materials and thermal barrier coatings which provide improved temperature capability and corrosion/oxidation resistance to the blade made from the base material. The blades also rotate at high speeds subjecting them to high stresses.
As a result of these extreme operating conditions, turbine hardware such as turbine airfoils develop operationally induced imperfections at predictable locations after operating for predetermined periods of time. The life may be extended by removing the hardware from service and refurbishing the hardware to remove these imperfections before the imperfections can grow to a size at which repair is not practical. Because of the high cost of fabricating turbine blades, refurbishment is a viable option if it can be accomplished at reasonable costs.
Currently, these operationally induced imperfections, such as cracks, typically appear in the coating at the fillet radius and the tip of turbine airfoils, although in certain cases, the imperfections may occasionally appear in other locations. Current repair procedures entail aggressive grit blasting of the coated portion of the blade to remove any cracks followed by fully removing the coating from the airfoil, which coating may be platinum aluminide (PtAl). The coating is applied from the platform axially outward to the airfoil tip, the underside of the platform and the root not being coated. Because of the high temperatures of operation, diffusion of the elements of the superalloy and the PtAl occurs. The first step in refurbishment of the airfoils is a substantially complete removal of the coating, which is accomplished by placing the airfoil in a chemical bath and stripping the coating from the blade to remove the cracks. However, because of the diffusion that occurs between the superalloy and the coating, the chemical strip process results in removal of material that is thicker than the thickness of the coating as originally applied. This material removal may result in airfoil wall thicknesses that are thinner than allowed, resulting in scrapping of the airfoil and replacement with a new airfoil. If the airfoil wall minimum thickness is maintained, additional repairs may be accomplished.
What is desired is a process that can remove damaged areas from the airfoils without thinning the airfoil walls, thereby reducing the scrap rate of airfoils. A feature that has been sought in the art is the ability to reapply coating into preselected areas without exceeding the maximum allowed coating thickness on the blade, particularly if coating the preselected areas can be accomplished without the time consuming step of masking, as excessive coating may lead to embrittlement, stress concentration and more cracking.